There are many known power assist steering systems for automotive vehicles. Some provide assist by using hydraulic power and others by using electric power.
Electric power assist steering systems that utilize a rack and pinion gear set provide power assist by using an electric motor to either (i) apply rotary force to a steering input shaft connected to a pinion gear, or (ii) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver's input torque applied to the vehicle steering wheel and/or (ii) vehicle speed. In U.S. Pat. No. 3,983,953, an electric motor is coupled to the input steering shaft and energized in response to the torque applied to the steering wheel by the vehicle operator. The electronic control system includes a torsion sensor and a vehicle speed sensor. A computer receives the output signals provided by both sensors. The computer controls the amount of the assistance provided by the motor dependent upon the applied torque and the vehicle speed.
U.S. Pat. No. 4,415,054, utilizes an electric assist motor having a rotatable armature encircling a steering member having rack teeth thereon. Rotation of the electric assist motor armature causes linear movement of the steering member through a ball nut drive arrangement. A torque sensing device is coupled to the steering column to sense driver-applied input torque to the steering wheel. The torque sensing device uses a magnet/Hall-effect sensor arrangement for sensing rotational deflection across a torsion bar. An electronic control unit monitors the signal from the torque sensing device and controls the electric assist motor in response thereto.
Previous electric assist steering systems have utilized discrete switches to apply and remove power to the electric assist motor. Such discrete switches typically permit arcing across the switch contacts prior to closure or during opening causing premature wear and subsequent failure of the contacts. To overcome problems associated with switch contacts, some motor control circuits have utilized solid state switching devices to connect operating current to the motor.
A solid state switching device is capable of passing a predetermined, maximum amount of current from a source potential to the power assist motor safely, i.e., without damaging the switching device itself. As the temperature of the solid state switching device rises, the maximum current that can be safely passed from the source potential to the power assist motor decreases. Circuits utilizing solid state switching devices have limited the drive current to the motor to protect the switching device by measuring the motor current. Limiting drive current only in response to measured motor current has not satisfactorily prevented failure of the solid state switching devices.
Electric power assist motors can draw a large amount of current during a steering maneuver. During such large current draws, electromagnetic radiation can occur causing electrical interference with other electrical controls. Electrical interference can cause spurious signals to be applied to the electric motor.
Still, other known power assist steering systems that utilize an electric assist motor have permitted power to be applied to the electric assist motor instantaneously with application of electric power to the vehicle. If a driver, with such a power assist steering system, had one hand on the steering wheel applying torque thereto while turning the ignition key with the other hand to start the vehicle, the driver would experience (i) the steering wheel rapidly turning in the direction of the applied torque the instant the ignition key is turned to the "on" position because the electric motor would apply maximum assist, (ii) no assist when the key is turned to the "start" position and (iii) maximum assist when the key is released to the "on" position. Such action of the steering wheel is quite startling to an individual accustomed to a hydraulic power assist steering system in which power assist is not available until the ignition key is turned to the start position cranking the engine and is very low until the engine "kicks-over" and reaches idle speed.